1. Field of the Invention
The present invention relates to a ceramic heater device and, more particularly, but not exclusively, to either a glow plug to be used for promoting the start of a diesel engine or a ceramic heater device to be used as a heater for igniting a petroleum fan heater.
2. Description of the Related Art
FIG. 10 of the accompanying drawings shows a ceramic glow plug 101 for a diesel engine as one example of the ceramic heater device of this kind. A rod (or column) shaped ceramic heater 2 is so fixed on the inner side (or in a column-shaped hole) of a metallic cylinder member 3 that the heater leading end 2a may protrude from the leading end 3a of the metallic cylinder member (hereinafter also called the xe2x80x9ccylinder memberxe2x80x9d) 3. These two members are so retained (or fixed) in a metallic body (hereinafter also called the xe2x80x9cbodyxe2x80x9d) 4 having a cylindrical shape as to protrude from the leading end 4a of the body. For this assembly, for example, the ceramic heater 2 is fixed gas-tight in the cylinder member 3 by fitting the ceramic heater 2 loosely in the cylinder member 3, by pouring a (not-shown) molten solder into the clearance, and by fastening the ceramic heater 2 by using the thermal expansion or cooling shrinkage of the cylinder member 3 by the poured solder layer 10. The assembly is completed by likewise fitting the cylinder member 3 integrated with the ceramic heater 2 loosely in the body 4 and by pouring the molten solder 10 into the clearance.
Here, the ceramic heater 2 is prepared by burying a (not-shown) heating member made of conductive ceramics and of a ceramic heating element or a high-melting point metal wire folded back (into a shape of letter xe2x80x9cUxe2x80x9d), in its portion close to the heater leading end 2a. At the two end portions of the U-shape of the heating member (or at the end portions of the two legs), moreover, terminals are disposed on the side faces of the ceramic heater 2 close to the rear end 2c through relay wires, and power feeding leads 15 and 16 are soldered to those terminals. The heater device thus constructed is able to generate a resistive heat to heat the ceramic heater 2 by feeding an electric current thereto through the power feeding leads 15 and 16.
In the structure of the prior device thus far described, the ceramic heater 2 fixed in the cylinder member 3 with the solder layer is subject to various external forces (e.g., an impact due to a fall or a bending force when it is mounted on the engine) in the subsequent manufacturing process or handling until the glow plug 101 is assembled. Therefore, the ceramic heater 2 may be cut (or broken) in the metallic cylinder member 3 along a thick line portion S, as shown in FIG. 10. However, this cut occurs in the metallic cylinder member 3 so that it cannot be visually confirmed from the outside. As a result, the structure may be assembled as it is in an engine (i.e., in a cylinder or an auxiliary combustion chamber) E.
During the combustion of the engine, on the other hand, the ceramic heater 2 is always exposed to a large temperature change (or a thermal shock) and a blast. When the engine is run, therefore, the metallic cylinder member 3 and the ceramic heater 2 are caused to relax (or become loose) therebetween by the difference in thermal expansion due to the temperature rise and/or due to vibration. If the relaxation occurs in the metallic cylinder member 3 having an inner circumference 3d of a constant internal diameter and a straight shape, on the other hand, the ceramic heater 2 is divided at the cut portion on the side of the leading end 2a, as shown in FIG. 11, to raise a problem that the cut portion may be separated and drop into the combustion chamber of the engine E.
Specifically, the soldered portions of the metallic cylinder member 3 and the body 4 have high and stable joint strength because the two members are made of metals. Although the joint strength between the inner circumference 3d of the metallic cylinder member 3 and the solder layer 10 is high, on the other hand, the joint strength between the outer circumference 2b of the ceramic heater 2 and the solder layer 10 is relatively low because they have just shrunk. Moreover, the ceramics and the solder have highly different coefficients of thermal expansion. Therefore, a relaxation (or looseness) easily occurs in the interface between the outer circumference 2b of the ceramic heater 2 and the solder layer 10. Especially in the case that the metallic cylinder member 3 is cut near the leading end 3a, its force for holding the cut portion of the ceramic heater 2 is so weak as to invite the separation or slide-out of the cut portion.
In another ceramic heater device, the ceramic heater is not fixed with the solder layer but is held by press-fitting it in the metallic cylinder member 3. In the case in which the ceramic heater has the aforementioned cut even if press-fitted, however, a problem arises in that the cut portion separates or slides out, as in the ceramic heater device using the solder layer. Independently of the solder layer structure or the press-fit structure, moreover, the cut of the ceramic heater may occur after it has been assembled in the engine, and a similar problem arises.
The present invention has been achieved in view of the aforementioned problems in the ceramic heater device such as the glow plug of the prior art, and an object of the invention is to prevent the cut portion of the ceramic heater from separating and sliding out from the structure in which the ceramic heater is fixed in the metallic cylinder member by soldering or press-fitting it therein.
Accordingly, the invention provides a ceramic heater device having a structure in which an axial ceramic heater is arranged in a metallic cylinder member so that its leading end protrudes from the leading end of said metallic cylinder member, wherein:
a convergent taper portion is formed at the leading end of said ceramic heater;
the leading end of said metallic cylinder member is disposed on the leading end side of the taper starting point of said taper portion;
said metallic cylinder member and said ceramic heater are fixed to each other with a solder layer interposed between their inner circumference and outer circumference respectively; and
at least a portion of said solder layer is also disposed on the leading end side of the taper starting point of said taper portion
By the aforementioned means, the solder layer is caused to exist on the leading end side from the taper starting point of the taper portion. This solder layer engages the taper portion to thereby prevent the ceramic heater from sliding out to the leading end side with respect to the metallic cylinder member even if the ceramic heater is relaxed in the metallic cylinder member. Even if the ceramic heater is cut on the rear end side of the taper starting point of the taper portion, for example, so that relaxation occurs in the interface between the outer circumference on the leading end side from the cut portion and the solder layer, more specifically, the solder layer existing on the leading end side from the taper starting point of the taper portion is thick on the surface of the taper portion. This thick portion engages the taper portion to thereby prevent the cut leading end portion of the ceramic heater from sliding out from the metallic cylinder member. Thus, in the case in which the present invention is embodied as the glow plug, the cut portion of the ceramic heater 2 is prevented from dropping into the auxiliary combustion chamber of the engine, even if relaxation occurs between the ceramic heater and the metallic cylinder member when the ceramic heater is assembled in the engine and run while having a cut in the metallic cylinder member. Preferably, the cone angle of the taper portion is properly set in the range of from about 10 minutes to 5 degrees.
According to a further aspect, the invention provides a ceramic heater device having a structure in which an axial ceramic heater is arranged in a metallic cylinder member so that its leading end protrudes from the leading end of said metallic cylinder member and in which said metallic cylinder member and said ceramic heater are fixed to each other with a solder layer interposed between their inner circumference and outer circumference respectively, wherein:
in said ceramic heater, a diametrically smaller portion having a smaller diameter than that of the remaining portion in said metallic cylinder member is formed at a portion located in said metallic cylinder member and corresponding to the portion proximate to the leading end of said metallic cylinder member; and
a solder layer is disposed at said diametrically smaller portion for preventing at least a portion of said ceramic heater from sliding out toward the leading end with respect to said metallic cylinder member.
The diametrically smaller portion may be either a straight portion formed straight toward the leading end, or a taper portion having a convergent taper shape. Here, the diametrically smaller portion in the present invention includes a constriction or a circumferential groove formed on the axis. The solder layer enters into the diametrically smaller portion to prevent slide-out at the time when the ceramic heater is liable to slide out to the leading end side from the metallic cylinder member.
Here, in any of the aforementioned means, the solder layer for preventing slide-out is constructed by the difference between the maximum and minimum external diameters of the ceramic heater at the portion proximate to the leading end of the metallic cylinder member, and this difference may be within a range of 10 microns to 300 microns. The slide-out preventing action is insufficient, if the difference is smaller than 10 microns. If the difference exceeds 300 microns, on the other hand, the molten solder is unable to spread over (or to bridge) the clearance between the inner circumference of the metallic cylinder member and the outer circumference of the ceramic heater by a capillary phenomenon, to thereby cause a danger that fixation with the solder layer fails.
According to a further aspect, the invention provides a ceramic heater device having a structure in which an axial ceramic heater is arranged in a metallic cylinder member so that its leading end protrudes from the leading end of said metallic cylinder member and in which said metallic cylinder member and said ceramic heater are fixed to each other with a solder layer interposed between their inner circumference and outer circumference respectively, wherein:
at least one recess is formed in the outer circumference of said ceramic heater at a portion located in said metallic cylinder member and corresponding to the portion proximate to the leading end of said metallic cylinder member, wherein a solder layer is disposed in said at least one recess for preventing sliding out of at least a portion of said ceramic heater toward the leading end with respect to said metallic cylinder member.
According to a further aspect, the invention provides a ceramic heater device having a structure in which an axial ceramic heater is arranged in a metallic cylinder member so that its leading end protrudes from the leading end of said metallic cylinder member, wherein:
a convergent taper portion is formed at the leading end of said ceramic heater;
said ceramic heater is press-fitted in said metallic cylinder member so that the taper starting point of said taper portion is positioned at a portion proximate to the leading end of said metallic cylinder member; and
the leading end of said metallic cylinder member converges at said taper portion.
With this construction, the rod-shaped ceramic heater can be arranged without being fixed with the solder layer so that the heater leading end may protrude from the leading end of said metallic cylinder member. In addition, the leading end of the metallic cylinder member converges at the taper portion so that it engages with the taper portion to perform the slide-out preventing action. Therefore, the leading end of the ceramic heater is prevented, even if cut, from sliding out from the metallic cylinder member, as described hereinbefore. With this structure, moreover, the ceramic heater device having the slide-out preventing action can be easily formed by press-fitting the ceramic heater with a suitable press-fit allowance into a predetermined depth of the metallic cylinder member.
According to a further aspect, the invention provides a method for manufacturing a ceramic heater device having a structure in which an axial ceramic heater is arranged in a metallic cylinder member so that its leading end protrudes from the leading end of said metallic cylinder member, comprising the steps of:
forming a convergent taper portion at the leading end of said ceramic heater; and
press-fitting said ceramic heater into said metallic cylinder member, starting with the leading end of the ceramic heater, to such a position that the taper starting point of said taper portion does not go beyond the leading end of said metallic cylinder member.